Wireless communication systems serving stationary and mobile wireless subscribers are rapidly gaining popularity. Numerous system layouts and communications protocols have been developed to provide coverage in such wireless communication systems.
Wireless communications channels between transmit and receive devices are inherently variable and their quality fluctuates. Specifically, the quality parameters of such communications channels vary in time. Under good conditions wireless channels exhibit good communication parameters, e.g., large data capacity, high signal quality, high spectral efficiency and throughput. At these times significant amounts of data can be transmitted via the channel reliably. However, as the channel changes in time, the communication parameters also change. Under altered conditions former data rates, coding techniques and data formats may no longer be feasible. For example, when channel performance is degraded the transmitted data may experience excessive corruption yielding unacceptable communication parameters. For instance, transmitted data can exhibit excessive bit-error rates or packet error rates. The degradation of the channel can be due to a multitude of factors such as general noise in the channel, multi-path fading, loss of line-of-sight path, excessive Co-Channel Interference (CCI) and other factors.
In mobile systems, signal degradation and corruption is chiefly due to interference from other cellular users within or near a given cell and multipath fading, in which the received amplitude and phase of a signal varies over time. The fading rate can reach as much as 200 Hz for a mobile user traveling at 60 mph at PCS frequencies of about 1.9 GHz. In such environments, the problem is to cleanly extract the signal of the user being tracked from the collection of received noise, CCI, and desired signal portions.
In Fixed Wireless Access (FWA) systems, e.g., where the receiver remains stationary, signal fading rate is less than in mobile systems. In this case, the channel coherence time or the time during which the channel estimate remains stable is longer since the receiver does not move.
Prior art wireless systems have employed adaptive modulation of the transmitted signals with the use of feedback from the receiver as well as adaptive coding and receiver feedback to adapt data transmission to changing channel conditions. Such adaptive modulation is applied to Single Input Single Output (SISO) and Multiple Input Multiple Output (MIMO) systems, e.g., systems with antenna arrays at the transmit and receive ends.
In both SISO and MIMO systems, however, the fundamental problem of efficient choice of the mode to be applied to the transmitted data remains. For general prior art on the subject the reader is referred to A. J. Goldsmith et al., “Variable-rate variable power MQAM for fading channels”, IEEE Transactions of Communications, Vol. 45, No. 10, October 1997, pp. 1218-1230; P. Schramm et al., “Radio Interface of EDGE, a proposal for enhanced data rates in existing digital cellular systems”, Proceedings IEEE 48th Vehicular Technology Conference (VTC'1998), pp. 1064-1068; and Van Noblen et al., “An adaptive link protocol with enhanced data rates for GSM evolution”, IEEE Personal Communications, February 1999, pp. 54-63.
U.S. Pat. No. 6,044,485 to Dent et al. teaches a transmission method and system which adapts the coding of data based on channel characteristics. The channel characteristics are obtained either from a channel estimation circuit or from an error feedback signal. In particular, when the signal-to-noise (SNR) characteristic is chosen as an indicator of the state of the channel, then a different coding is applied to the data being transmitted for high and low SNR states of the channel. In addition, the encoding also employs information derived from the cyclic redundancy check (CRC).
The method taught by Dent varies the coding rate only and not the modulation rate. This method does not permit one to select rapidly and efficiently from a large number of encoding modes to adapt to varying channel conditions.
U.S. Pat. No. 5,559,810 to Gilbert et al. teaches a communication system using data reception history for selecting a modulation technique from among a plurality of modulation techniques to thus optimize the use of communication resources. At least one block of data is transmitted with a particular modulation technique and a data reception history is maintained to indicate transmission errors, e.g., by keeping a value of how many blocks had errors. The data reception history is updated and used to determine an estimate of transmission signal quality for each modulation technique. This value is then used in selecting the particular modulation technique.
The system and method taught by Gilbert rely on tracking errors in the reception of entire blocks of data. In fact, signal quality statistics, e.g., signal-to-interference and noise ratio (SINR) are derived from the error numbers for entire blocks of data, which requires a significant amount of time. Thus, this method does not permit one to select rapidly and efficiently from a large number of encoding modes to adapt to varying channel conditions, especially in the event of rapid fades as encountered in mobile wireless systems.
It would be an advance to provide a mode selection technique which allows the system to rapidly and efficiently select the appropriate mode for encoding data in a quickly changing channel. It is important that such technique be efficient in all wireless systems, including Multiple Input Multiple Output (MIMO), Multiple Input Single Output (MISO), Single Input Single Output (SISO) and Single Input Multiple Output (SIMO) systems as well as systems using multiple carrier frequencies, e.g., OFDM systems.